The present invention relates to an apparatus and method for detecting the level of a liquid in a vessel.
In a specific aspect the present invention relates to an apparatus and method for detecting the level of a liquid in storage tanks, which may or may not be superatmospherically or subatmospherically pressurized.
Numerous methods have been used to determine liquid levels in storage vessels. However, these methods have the drawback that they only provide rough measurements which are often unacceptable.
One gauging method presently employed in certain liquid storage tanks, such as gasoline station storage tanks, is referred to as the bubbler system. In this system compressed gas, typically air, is introduced in very small quantities into a tube which runs from a metering area to the gasoline tank where the tube is extended vertically through the tank to within an inch of the bottom of the tank. The air bubbles out of the bottom of the tube, displacing the gasoline and the resultant pressure in the tube is proportional to the depth of the fluid in the tank. This pressure is sensed by a pressure gauge which provides a readout indicating the depth of gasoline. The bubbler system, however, is inadequate in that it does not compensate for changes in liquid density and liquid temperature which in turn affects the liquid vapor pressure thereby affecting the liquid level readout. Further this system presents obstacles in pressurized or sealed tanks when the bubbler gas has to vented due to pressure buildups and the bubbler gas is commingled with vapours such as hydrocarbons. Also, any long-term electronic drift associated with electronic pressure sensors is not accounted for in the subject method. Additionally, the bubbling of a gas through the hydrocarbon or other liquids may be deleterious to its quality, i.e., the degradation caused by oxidation of certain hydrocarbon constituents by contact with air.
Along the same vein, U.S. Pat. No. 4,199,983 (Kobayashi et al.) discloses an apparatus for detecting the level of fuel oil in a vessel. The subject apparatus comprises a sensing pipe having a sensing hole in its lower portion which pipe is continuously supplied with a gas. The pressure in the sensing pipe continuously increases as the level of fuel oil is raised above the level of the sensing hole. A fluid pressure-responsive device is connected to the sensing pipe. The pressure in the sensing pipe is indicative of the level in the tank. The subject device also does not compensate for changes in temperature which in turn affects the fuel oil density.
With the advent of self-service gasoline stations, there has been a decline in full-time attendants employed specifically to sell gasoline and automotive products. Thus, modern gasoline stations are increasingly operated by only a cashier who also dispenses food. The old practice of lowering a calibrated pole into the storage tank to determine liquid level is prohibited by laws which preclude personnel who dispense food from handling fuel in any way.
In the case where a liquid storage tank is pressurized, the tank must first be depressurized prior to insertion of a calibrated pole. Unfortunately, such a technique is tedious and inefficient when a large number of tanks are involved. Also, such a technique is not amenable to automatic telemetering of vessel inventories.
Further, environmental concerns have engendered the need for a method and apparatus for precisely detecting liquid levels in underground tanks where leakage into the surrounding groundwater is a possibility. A precise liquid level measurement technique would allow one to ascertain the existence of an underground tank leak, facilitating compliance with environmental regulations.
Measuring a liquid level in a closed pressure vessel accurately also presents specific problems. For instance, capacitance systems wherein the level in a pressurized tank is determined as a function of the capacitance measured by a probe in the tank are unavoidably inaccurate when debris or other non-liquid foreign matter adheres to the capacitance probe. Further, the oscillator associated with the capacitance probe is subject to drift.
Also, these capacitance devices must be calibrated for specific fluid dielectric constants and are generally unstable in gasoline-alcohol-water mixtures.
A liquid level measuring device should advantageously be adaptable to remote telemetering whereby vessel inventories can be precisely monitored and capacities judiciously managed. The term telemetering as used herein refers to the transmission of the measurement, i.e., liquid level to any point remote from the primary detector by any means, e.g. electrically, telephonically, radio waves.
Further, float and mechanical devices suffer from mechanical friction and sticking problems. Pneumatic devices must be calibrated for the specific gravity of the fluid being measured and are not easily adjusted to compensate for temperature variations in the liquid being measured.
Accordingly, it is an object of the present invention to provide a method and apparatus for accurately measuring the liquid level in a vessel at any pressure, whereby any changes in liquid density due to temperature or composition changes are compensated for and the longterm drift effect associated with an electronic pressure sensor means is also compensated for.
Further, it is an object of the present invention to provide a method and apparatus for measuring liquid levels whereby the measurement is amenable to remote telemetering.
These and other apparent objects and advantages are obtained by the present invention described in more detail in the following specification and accompanying drawing.